Haven't manuscripts been found since the KJV was translated in 1611?
Yes. What's more, the vast majority of them vindicate KJV readings.
There are currently over 5000 extant Greek manuscripts of the New Testament in one form or another (miniscules, unicals, lectionaries). Approximately 99% of them agree with each other and the KJV; the remaining 1% disagreeing with themselves as often as the majority. The four centuries since the KJV was translated have provided no "deluge" of new material. In fact, almost every non-KJV reading found since 1611 was known before 1611 and were rejected by the KJV translators.
When a critic insists that "recent finds" demand more revisions, he is misleading you. The vast majority of textual evidence found since 1611 disagrees with modern critical readings! This is not to say that a majority reading is the sole consideration in selecting a variant, but it does show that modern scholarship scrapes the barrel routinely to find reasons to sell you a new Bible every year.
For more details, see Which Version is the Bible? by Floyd Nolen Jones, chapter 3.The following is from Sam Gipp's The Answer Book.
QUESTION: I've heard that there have been many manuscripts discovered since 1611 that the King James translators didn't have access to. Do these strengthen or weaken the King James Bible?
ANSWER: They strengthen the King James Bible.
EXPLANATION: There have been many manuscripts found since 1611, but there have been no new READINGS found.
Many critics of the Word of God have used the argument of "new evidence" that the King James translators didn't have as a basis to degrade its authority. The fact is, the King James translators had all of the readings available to them that modern critics have available to them today.
One of the most prominent manuscripts which has been discovered since 1611 is the Sinaitic manuscript. This witness, though horribly flawed, was found amongst trash paper in St. Catherine's monastery at the foot of Mt. Sinai in 1841 by Constantine Tischendorf.
Sinaiticus is a sister manuscript of the corrupt manuscript Vaticanus. Both read very similarly. So, although the Sinaitic manuscript was discovered over 200 years after the Authorized Version was translated, its READINGS were well known to the translators through the Vatican manuscript which was discovered in 1481 and also through the Jesuit Bible, an English translation of 1582.
So we see that there are no readings available today to scholars which were not already in the hands of the King James translators. We might further add that an honest scholar will admit that this "great number of newly discovered manuscripts" that are trumpt abroad agree with the Greek text of the Authorized Version rather than challenging it.